Nowadays, radio communication systems, such as a cellular phone system, a wireless LAN (Local Area Network) and the like, are widely used. Moreover, in the field of radio communication, next-generation communication technology is discussed continuously in order to further improve communication speed and channel capacity. As the next-generation communication technology, for example, LTE or LTE-Advanced standardization is completed or under way.
Some of the radio communication technology may establish radio communication between, for example, a radio base station apparatus (hereinafter referred to as a “base station”) and a terminal apparatus (hereinafter referred to as a “terminal”) by using a plurality of carriers. The radio communication established by using the plurality of carriers may be referred to as carrier aggregation. Incidentally, the carrier means one frequency band defined by a bandwidth and a center frequency, for example.
It is also possible for the base station to transmit a control signal to the terminal, for example, in the radio communication using the carrier aggregation. In this case, the control signal is transmitted by using a downlink control channel (PDCCH: Physical Downlink Control CHannel). Information about radio resources, information about a modulation and coding scheme (MCS: Modulation and Coding Scheme) and the like are contained in the control signal.
The information about the radio resources includes, for example, information of the radio resources (time and frequency, for example) allocated to an uplink shared channel (PUSCH: Physical Uplink Shared CHannel) and a downlink shared channel (PDSCH: Physical Downlink Shared CHannel) and the like. Further, the information about the modulation and coding scheme includes, for example, information about a coding rate and a modulation system with respect to a data signal (hereinafter referred to as “data”) to be transmitted, information about a demodulation system and the coding rate with respect to the received data and the like.
The control signal is generated by the base station as needed. Based on the control signal transmitted from the base station, the terminal can receive and demodulate the data transmitted from the base station, and modulate the data and transmit it to the base station.
Meanwhile, when the base station establishes the radio communication by using the carrier aggregation, it transmits the control signal by using either one of the plurality of carriers. FIG. 25A and FIG. 25B are views respectively illustrating configuration examples of radio frames in a downlink direction. In this example, a base station #1 transmits the control signal by using a carrier #4, out of a carrier #3 to a carrier #5, and a base station #2 also transmits the control signal by using the carrier #4.
Incidentally, in the example of FIG. 25A and FIG. 25B, the radio base station #1 can transmit the data by using the three carriers (carrier #3 to carrier #5). In addition, the radio base station #2 can also transmit the data signal by using the three carriers (carrier #3 to carrier #5).
Moreover, FIG. 25C is a view illustrating a state where the control signal is transmitted from the two base stations #1 and #2 by using the radio frames formed according to FIG. 25A and FIG. 25B.    Non-Patent Literature 1: 3GPPTS 36.211 V10.0.0 (2010-12)    Non-Patent Literature 2: 3GPPTS 36.212 V10.0.0 (2010-12)    Non-Patent Literature 3: 3GPPTS 36.213 V10.0.1 (2010-12)    Non-Patent Literature 4: 3GPPTS 36.313 V10.0.0 (2010-12)
However, when the plurality of base stations use the same carrier and transmit the control signals, the control signals transmitted from the plurality of base stations may interfere with each other. In the example of FIG. 25A to FIG. 25C, for example, the two base stations #1 and #2 transmit the control signals by using the same carrier #4. At this time, for example, the control signal from the base station #1 and the control signal from the base station #2 may interfere with each other, and the terminal may be unable to receive the control signal from the base station #1. When the terminal is unable to receive the control signal transmitted from the base station #1, it can hardly receive the data transmitted from the base station #1 nor transmit the data to the base station #1. Namely, the terminal is unable to establish the radio communication with the base station #1.
Meanwhile, when the terminal establishes the radio communication with the base station #2, it is also possible that the terminal is unable to receive the control signal transmitted from the base station #2 due to interference of the control signal transmitted from the base station #1. In this case, it is difficult for the terminal to establish the radio communication with the base station #2, as the terminal is unable to receive the data transmitted from the base station #2 nor to transmit the data to the base station #2.